All modern ships are equipped at the bow with voluminous Delta bulbs adapted to reduce resistance to their advancement.
The Delta bulb is a system for generating a bow wave in phase opposition to that generated by the ship in such a way that the resulting wave is as small as possible. The reduction in height of the ship wave makes it possible to greatly reduce the resistance to its advancement.
Delta bulbs must therefore create a wave of a comparable size to that of the ship. To be effective a Delta bulb must have a considerable volume and very full and rounded shape of the front at the waterline.
A large Delta bulb suitable to generate a wave able to lower that generated by the ship has a geometry which behaves very well in ice-free waters, but which is entirely unsuitable for navigation in icy waters.
A Delta bulb has, in fact, an almost completely flat front surface which does not permit the crushing of the icy surface and thus the navigation through ice. Even if the ship has a powerful propulsion system the flat surface of the Delta bulb arranged near the waterline would hit the ice with a very extensive area, and thus with a very low specific pressure, which does not permit breaking of the ice. In addition, in the bow area there would be an ice build-up which would gradually block the advancement of the ship.
The Delta bulb cannot therefore be used when a ship also needs to operate in icy waters.
As a result, in ships intended for navigation in icy waters, the need to ensure adequate manoeuvrability has led to a sacrifice of propulsive performance in open water, forgoing the Delta bulb.
A widespread solution on ships intended for navigation in icy waters provides for the use of icebreaker bulbs. These bulbs have a sharp form ideal for breaking ice, but unable to offer comparable hydrodynamic performances to those of a Delta bulb.
In the U.S. Pat. No. 3,521,590 it has been proposed to adapt the forms of a traditional bow bulb for navigation in open water so as to make it suitable for crushing ice. Traditionally bow bulbs for navigation in open water have a slightly flattened cross-section in a vertical or circular direction. In U.S. Pat. No. 3,521,590 it is proposed to flatten the bulb in a horizontal direction. Considering a transversal cross-section, the bulb should have a greater extension in width than the extension in height. It is proposed, in addition, to add a blade which extends along the front profile of the bulb from the tip of the bulb as far as the stem bow which it connects to. The bulb is also modified so as to have a V-shape suitable to divert the crushed ice along the sides of the ship in order to push it to the sides of the ship instead to under the hull.
The proposal of U.S. Pat. No. 3,521,590 is a compromise solution, which requires a sacrifice in terms of open water hydrodynamic performances. The bulb shapes have in fact been changed significantly, away from the best shapes for ensuring the desired hydrodynamic effects. As regards navigation in icy waters the solution proposed in U.S. Pat. No. 3,521,590 solution does not appear to be particularly high performance. In fact, the deviation of the crushed ice along the sides may result in an excessive build-up of ice on the sides in many operational situations. The result is a slowing of the movement of the ship and in some extreme cases, its blockage.
The need continues to be deeply felt therefore in the industry of ship intended for navigation in icy waters to offer high propulsive performance in open water, but without sacrificing adequate manoeuvrability in icy waters.